The invention relates to a process for treating a residual product which contains sodium chloride and sodium sulphate and which occurs in the form of a crystallised salt mixture upon evaporation and cooling of a sodium hydroxide solution obtained from a diaphragm electrolysis.
Known processes for the treatment of a residual product of the type described provide for dissolving out the sodium sulphate by slurring the residual product in water or in a sodium chloride brine with a low sulphate content, preferably at a temperature of the slurry at which a maximum of sodium sulphate and a minimum of sodium chloride become dissolved. Thereafter, the sodium chloride which has remained undissolved in the slurry is removed by centrifuging. The separated mother brine can be recovered if the sodium sulphate is precipitated as decahydrate (Glauber salt) by cooling to about 0.degree. C, or if a precipitation of calcium sulphate is caused by addition of calcium chloride.
A relatively expensive equipment is necessary for these known processes, and in addition the cost for energy is considerable, especially in those cases where the proportion of sodium sulphate to be separated out is relatively small. The, as a secondary product obtained sodium sulphate is in the form of the difficult depositable Glauber salt.